Animation Alley - Balance

Hello everybody. Before we discussed posing. This time we will
go over the fine points of balance. Actually balance is related to
posing, however, as posing is such a broad subject, we will take
each element separately, this week's lesson revolves around
balance. Balance is very important when it comes to posing
characters, because it makes the poses appear natural and
believable. A drawing doesn't have any weight, so we have to
simulate that weight. In a drawing we have the freedom to break any
rule that we want, for example; you can make your character stand
on a 45 degree angle, and it will never fall, because gravity
doesn't affect it. However, everybody will see that, in reality,
that pose would be impossible to achieve. Lets start with the
center of mass, and the center of gravity. Center of mass is the
place where all the body mass could be gathered and gravity would
still affect the body in the same way. In a body of constant
density, the center of mass would be located in the center of the
object. Center of gravity is the spot where you could apply all of
the gravitational force and yet the body would react in the same
way. On rigid bodies, with constant density, the center of mass and
center of gravity are located on the same spot. However, we are
talking about characters, so the position center of gravity changes
depending on the pose.

Above is a character in a "hero pose." The arrow on his abdomen
would be the approximate location of the center of gravity. We
really don't care about the center of mass, because in the end what
affects the body is the gravity. A figure has a good balance if
both feet are on the sides of the center of gravity. That means
that the character will not fall. Let's see another example:

In the above case, the contact point and the center of gravity are
not in equilibrium (more about this later). This means that the
character would actually fall forward. To explain this, we need to
understand "momentum" or "torque." Torque consists on a force
applied on some point of the body other than the pivot, and the
distance between that point and the pivot. This force causes a
rotational effect that would make the character fall forward in
this particular case. Try to stand in the same position as the
figure in the above image, and see what happens. You'll be lucky if
you don't break your nose trying. Notice, in this case, the center
of gravity was not located within the abdomen, like the other case.
For artists like us, finding the location of the center of gravity
is rather subjective. You can obviously try to find it using
mathematical formulas, but that can be very complicated, especially
on characters. So, you really have to use your common sense for
this; imagine the weight of every body part then ponder those
weights. If, for example, the character on that picture had a big
head, the center of gravity would be located somewhere around the
neck. Another thing to consider is when the character is carrying
something. Every object of your scene has its own center of
gravity. When a character carries something, both centers of
gravity make a new center of gravity, which is located somewhere
between the first two called "average center of gravity." You can
see an example in the next image.

Are we allowed to break the balance? Absolutely. However, we have
to know when we can do it and why. We can make static poses as well
as dynamic poses, where we can portray: characters walking,
running, falling, and dancing. An animated character is in a
constant "balance off balance," during the flow of the action.
For example, let's take a look at a walk:

When the feet are on the ground, we have a good balance. When we
are on the highest position (like the character featured in the
above image), we are off balance, and we don't fall because the
front foot catches our body. So, you can make off balance poses as
well, but you have to know why and when. If you want to further
give the impression of movement you can add motion blur but
that's a different subject. It's been a good thing to come back to
these little articles. I hope you found this useful. Happy posing,
and keep on animating! nemircAnimation Alley is a regular featured
column with Renderosity Staff Writer Sergio Rosa [nemirc].Sept. 12, 2005